Pressure sensitive switch



" oa. 2o, 1970 E. BAHNIUK PRESSURE SENSITIVE SWITCH Filed Jan. 29, 1968 .von ,r M vv V. w B4 frana/Ks 3,535,480 PRESSURE SENSITIVE SWITCH Eugene Bahniuk, Gates Mills, Ohio, assignor to The Weatherhead Company, a corporation of Ohio Filed Jan. 29, 1968, Ser. No. 701,439 Int. Cl. H01h 35/40 U.S. Cl. 200-83 15 Claims ABSTRACT OF THE DISCLOSURE A -wide differential pressure switch for use in air conditioning systems or the like to indicate loss of pressure. The switch opens at a relatively low gauge pressure in the order of 3 to 5 p.s.i. and closes at a gauge pressure in the order of 35 p.s.i. The mobile switch contact is mounted on a snap element operated by a spring biased plunger through a lost motion drive. The lost motion provides relatively wide differential pressure of operation with a relatively low rate spring. The 4lost motion drive also permits the snap element to snap freely through its full snap travel.

BACKGROUND OF INVENTION This invention relates generally to pressure-operated switching devices and more particularly to a pressure switch adapted for use in refrigeration systems or the like to prevent system operation when insucient charging fluid is present in the system for proper operation.

In many refrigeration systems at least part of the compressor lubricant passes through the system with the refrigerant. Consequently the compressor can be damaged "United States Patent O due to lack of lubrication if it is operated when the syst tem is not properly charged with refrigerant. Therefore, it is desirable to provide an automatic cut-olf device to prevent system operation when the system is not properly charged.

The pressures in the various portions of a refrigeration system, such as the system used in air conditioning, vary through relatively wide limits depending upon the environmental conditions and the operating conditions of the system. For example, during normal operation the portion of the system between the compressor outlet and the expansion valve is at a relatively high pressure and the portion of the system between the expansion valve and the compressor inlet is at a relatively low pressure. The particular pressures reached by the high and low pressure portions of the system vary with environmental and operating conditions. On the other hand, when the system is not operating, the pressure throughout the system tends to equalize at a pressure determined by the environmental temperature.

A cut-olf device which senses lack of proper charge, therefore, must be arranged so that it does not cycle during the normal cyclic operation of the system and so that it does not provide a false signal to prevent proper operation of the system when suiiiicient charge is present.

SUMMARY OF INVENTION The present invention provides a pressure responsive switching device wherein the switch opens at a relatively low pressure condition and does not reclose until the pressure being sensed is at a value substantially higher than the pressure required to open. The switching device may, therefore, be used in a refrigeration system or the like to prevent operation of the refrigeration system in the event that leakage causes a loss of the refrigerant charge.

In the illustrated embodiment the switching device may be set to open at a gauge pressure in the order of 3 to 5 p.s.i. and will not reclose until the pressure being sensed is in the order of 35 p.s.i. Further the device is arranged so that it is not damaged by pressures in the order of 700 p.s.i. A switching device may, therefore, be used in the low pressure portion of a refrigeration system to prevent system operation in the event that proper refrigerant charge is not present in the system.

The illustrated embodiment includes a diaphragm operated plunger which is resiliently urged in one direction by a calibration spring and which is connected through a lost motion device to a snap element. The snap element carries a mobile contact of a switch and moves between two positions of stability in response to plunger movement. The lost motion connection between the snap element serves two functions. First it provides for substantial plunger travel between the two operating positions so that excessive spring rates are not required even when large differential pressures are provided. Second it permits rapid snap action of the snap element by minimizing the mass of the system involved in the snap action.

OBJECTS OF INVENTION It is an important object of this invention to provide a novel and improved pressure sensitive switching device; wherein the two pressures of operation are relatively widely` spaced; wherein the lower operating pressure may be a relatively low pressure; and wherein the device is capable of withstanding relatively high over-pressure.

It is another important object of this invention to provide a novel and improved pressure sensitive switching device adapted to be used in refrigeration systems or the like to indicate the absence of a proper refrigerant charge.

It is another important object of this invention to provide a novel and improved pressure responsive switching device including a pressure responsive plunger mechanism connected through a lost motion mechanism to a snap element.

Further objects and advantages will appear from the following description and drawings wherein:

FIG. l is a schematic illustration of a typical air conditioning system incorporating a switching device in accordance with this invention;

FIG. 2 is a longitudinal section of a switching device incorporating this invention illustrating the position the elements assume when the pressure being sensed is at the low operating value;

FIG. 3 is an exploded perspective view of the elements of the switching mechanism; and,

FIG. 4 is a fragmentary section similar to FIG. 2 illustrating the position the elements assume at the upper operating pressure.

FIG. 1 schematically discloses a typical automotive air conditioning system with a cut-olf switching device incorporating this invention. The system includes a compressor 10 having an outlet 11 connected to a condenser 12. The refrigerant leaving the condenser 12 ows through an expansion valve 13 to an evaporator 14 and back to the inlet 16 of the compressor. When` the system is operating, the pressure in the portion of the system between the outlet 11 and the expansion valve 13 is sufficiently high so that the condenser functions to liquify the refrigerant. In the 10W pressure portion of the system, between the expansion valve 13 and the inlet 16, the pressure is such that the refrigerant Vaporizes inthe evaporator 14. When the compressor is shut off during the cyclic operation of the system, the pressure throughout the system tends to equalize at a pressure determined by the environmental temperature.

In a typical air conditioning system the pressure extremely cold weather the pressure of the non-operating System may drop to substantially Zero gauge pressure, but air conditioning does not operate at that time so that operation of the cu-t-or` switch would not be of any importance.

The compressor 10 is driven by an electric clutch 17 from the vehicle engine. A cut-off switch 18 incorporating this invention is connected to the system in the low pressure portion thereof between the expansion valve and the 'inlet 16. The switch may be installed anywhere in thel low pressure circuit. The cut-off switch 18 is provided with contacts connected in the power circuit of the clutch 17 in the case of automotive installations or may be connected in the circuit of an electric Imotor if such a power source is used to drive a compressor. When the switch is closed, the refrigeration system can cycle in a normal irnanner. However, when the switch of the cut-off switch 18 opens, operation of the compressor and, in turn, of the refrigeration system cannot occur. The switch is arranged to open only when the pressure in the low pressure portion of the system indicates that insuicient charge is available for proper operation. Therefore, if leakage occurs from the system for any reason, the pressure in the low pressure portion of the circuit will drop, causing the switch of the cut-off 18 to open and terminate further operation. This prevents damage to the mechanism of the system by preventing operation without proper lubrication.

In normal operations the pressure at which the system equalizes when the compressor is not operating is higher than the pressure present in the low pressure phase of the circuit during operation. Therefore, the switch 18 is provided with a substantial differential in pressure between its pressure for opening and its pressure for closing. In one embodiment the switch 18 is set to open when the pressure sensed drops to the pressure in the order of 3 to p.s.i. The switch will not reclose unless the pressure returns to a pressure higher than about 35 p.s.i. iFurther 'the switch will not open even though the pressure in the low pressure portion of the circuit drops well below 35 p.s.i. so long as it is above the opening pressure of 3 to 5 p.s.i. so cycling will not occur so long as the system is properly charged.

Reference should now be made to FIGS. 2 through 4 rfor a clear understanding of one embodiment of a pressure responsive switch mechanism in accordance with this invention. This embodiment includes a housing 21 having a threaded nipple 22 formed at one end so that the switch :may Ibe connected to a refrigeration system or the like. The housing is provided with a tubular skirt 23 which defines a cylindrical switch cavity 24 extending upwardly from a radial shoulder 26. Extending from the shoulder 26 in a direction toward the nipple 22 is a cylindrical 'bore 27 which extends to an annular groove 28 proportioned to receive a soft elastomeric diaphragm 29.

The diaphragm is preferably formed with a lip seal 31 so that a proper seal is provided at all pressure encountered during the operation of the system. The diaphragm 29 is retained in position by a support ring 32 which closely ts the inner wall of the skirt 23 and engages the shoulder 26. Positioned above the ring 32 is a switch. body 33 formed with an axial projection 34 which lits into a bore in the ring 32 to radially locate the lower end of the switch body 33. The upper end of the switch body 33 abuts an upper support and terminal ring 36 which closely fits the skirt 23 and is axially positioned against the switch body by an inturned flange 37 at the open end of the skirt 23. Mating axially extending surfaces 38 and 39 cooperate with the projection 34 to radially position the switch body.

The exterior surface 41 of the switch body 33 is cylindrical and sized so that it fits within the skirt 23 with clearance. The terminal ring 36 and the ring 32 are formed of nonc'onducting materials so that the switch body 33 is electrically insulated from the body 21.

A pair of terminals 42 and 43 extend through the terminal ring 36 and are anchored therein. The lower end of the terminal 42 is connected to the switch body 33 by an angle clip 44. The inner end of the terminal 43 is connected to one end of a cantilever mounted contact support arrn 46. The contact support arm 46 is shaped as best illustrated in FIG. 2 and provides a central opening 47 and a contact 48 which constitutes one of the contacts of the switch mechanism.

A snap element 49 is mounted in the switch body 33 as best illustrated in FIG. 3. The switch body is formed with a pair of opposed, V-sha-ped grooves 51 which receive the opposite ends 52 of the snap element 49 and provide pivot type mounting for the snap element. 'Ihe spacing between the grooves 52 and the length of the snap element `49 is arranged so that the snap element is deformed from its unstressed condition to one or the other of its two positions of stability illustrated in FIGS. 2. and 4. The snap element 49 is provided with a central opening 53 and is symmetrically shaped around the opening 53 so that the lateral cross section is substantially constant throughout its length. With this arrangement the snap element tends to form a relatively smooth arch in each of its positions of stability.

A second contact 54 is mounted between the central opening 53 and one end 52 in a position to engage the contact 48 when the snap element is arched upwardly in the position of stability of FIG. 4. When the snap element is in the downwardly arched position of stability of FIG. 2, the two contacts 48 and 54 are spaced from each other. Proper orientation is provided between the terminal ring 36 and the switch body 33 by the mating noncircular surfaces 38 and 39.

A plunger mechanism is provided to snap the snap element 49 between the two positions of stability in response to movement of the diaphragm 29. The plunger mechanism includes a stem element 56 which extends with clearance through a bore 57 in the switch body 33 from a flanged end 58. The end 58 provides a flat end surface 59 which engages the upper side of the diaphragm 29 and extends outwardly to a position in close proximity to the opening A61 in the ring member 32. There-fore, the diaphragm is substantially completely conned.

The stem 56 is provided with a conical shoulder 62 which extends inwardly to a cylindrical portion 63 which lits through the opening 53 with clearance. At the upper end of the cylindrical portion 63 is a radial shoulder 64 from which projects a cylindrical projection 66. A ring member 67 fits over the projection 66 and is axially positioned by the shoulder 64. The ring 67 is provided with a conical end 68 which cooperates with the opposed conical surface 62 to form a lost motion connection between the stem 56 and the snap element 49. Extending upwardly from the ring 67 and radially positioned by a skirt 69 is a coil spring 71. The upper end of the spring 71 engages a calibration screw 72 threaded into the terminal ring 36.

The nipple 22 is provided with an axial bore 73 extending to a chamber 74 formed in part by the lower surface of the diaphragm 29. When the pressure in the chamber 74 is an atmospheric pressure or at pressures approaching the cut-off pressure, the elements are in the position of FIG. l. In the illustrated embodiment the cut-off pressure is in the order of 3 to 5 p.s.i. The value of the cutoff pressure can be changed by adjustment of the calibration screw 72 which in turn adjusts the resilient force of the spring 71 on the stem 56.

As the pressure in the chamber 74 increases, the stem 56 moves upwardly against the action of the spring 71 and the action of the snap element 49 through a distance which is a function of the increase in pressure and the spring rate of the system. Preferably the diaphragm 29' should be formed of relatively soft material so that it provides a relatively low spring rate and does not signiiicantly enter into the overall spring system when it is deflected from its unstressed condition of FIG. 2. However, if the spring eifect of the diaphragm 29 is signicant, corresponding adjustment in the spring rates of the snap element 49 and the coil spring 71 may be made.

The rate of the spring 71 is relatively constant with increasing force being exerted on the plunger 51 as the plunger moves toward the spring. However, the spring rante of the snap element 49 approaches a sinusoidal curve with the resilient reaction force applied to the stem 56 increasing from zero as the snap element is deflected from one position of stability to a maximum force and then dropping through zero when the snap element 49 approaches the mid position of instability. Therefore, as the plunger 56 moves upwardly, the resulting resilient forces of both the spring 71 and the snap element 49 increase until the snap element reaches a position of maximum force. Continued upward movement toward the position of instability of the snap element causes further increase in the force of the spring 71 while the force of the snap element 49 decreases to zero. As soon as the stem 56 carries the snap element .through its position of instability, it snaps almost instantaneously to the position of FIG. 4. Suicient lost motion is provided so that the snap element is unrestrained in this movement. Consequently the mass of the plunger system need not move with the snap element and a rapid or almost instantaneous movement results.

When the switch is closed as illustrated in FIG. 4, the flange 58 engages the underside of the switch body 33 to prevent further extension of the stem. Consequently the switch mechanism is not subjected to excessive forces even when the pressure in the chamber 74 reaches relatively high values, considerably above the pressure required to close the switch. Since the diaphragm is thick with respect to the clearance between the surface 59 and the support ring 32, it is not damaged by such overpressure.

As the pressure in the chamber 74 drops below the pressure required to maintain the elements in the position of FIG. 4, the spring 71 extends causing the stem 56 to move toward the chamber 74. As the spring 71 extends, its force on the stem drops. However, as the snap element 49 is deformed from the position of stability of FIG. 4, it produces an increasing reaction force tending to resist the action of the spring 71. The reaction force of the snap element increases until a maximum force is reached and continued movement toward the position of instability then causes a decrease in the force of the snap elements 49 until the position of instability is reached. Because the snap element 49 resists the action of the spring 71, the force produced by the spring 71 must be greater than the maximum force of resistance of the snap element.

Once the snap element passes through the position of instability, it snaps to the contact open position of FIG. 2. This action occurs almost instantaneously because the lost motion connection between the stem and the snap element permits free movement of the snap element. In order to insure that the contacts 48 and 54 snap open, the various elements should be proportioned so that the opening occurs after the snap element 49 passes through the position of instability in a direction toward the contact open position of FIG. 2. When this arrangement is provided, arcing is minimized by the rapid snapping movement of the contact 54.

If the spring rate of the spring 71 and the diaphragm 29 is greater than the negative spring rate of the snap element 49, the closing of the contacts will occur slowly since contact closure occurs before the position of instability of the snap element 49 is reached. In the illustrated installation of the switch, this does not present a problem since the pressure in the low pressure portion of the refrigeration system is highest when the compressor is not operating so closure of the contacts normally occurs only when the system is shut off. On the other hand,

in some installations it may be desirable to provide snap action on both closing and opening. In such instances snap elements should be selected to have a negative spring rate greater than the positive spring rate of the remaining springs in the system and so that the contacts close as the force of the snap element is dropping from the maximum toward zero. With such an arrangement a rapid closing operation occurs. With the illustrated embodiment welding of the contacts is not a severe problem, since there is a rocking motion between the two contacts as the contacts commence to move toward an open position. This rocking movement breaks any welds which may be present and insures proper switch operation.

The lost motion connection provided between the stem and the snap element serves a dual function. It provides for substantial movement of the stem between the two positions at which the switch operates so excessively high spring rates need not be required even when relatively wide differential pressures are desired. Further it allows the contacts to snap open with an almost instantaneous action since the mass involved in the snapping movement is only the mass of the snap element. It is preferable to also arrange the snap element so that it produces a relatively large reaction force when large dierentials are required since the force of the snap element is reversed between opening and closing and as- Sists in providing substantial differential pressures for operation. It should be understood that a pressure responsive switching device incorporating this invention may be used in many other applications and that the invention is not limited to the use of such a device in a refrigeration system.

Although a preferred embodiment of this invention is illustrated, it is to be understood that various modifications and rearrangements of parts may be resorted to without departing from the scope of the invention disclosed and claimed herein.

I claim:

1. A pressure responsive switching device comprising a body, a snap element mounted in said body, said snap element having two positions of stability and a substantially sinusoidal spring rate between said positions of stability with a force reversal occuring in a, position of instability, pressure responsive plunger means in said body movable between first and second positions, said plunger means including spring means resiliently operable to urge said plunger means toward said rst position with a force which increases as said plunger moves from said first position to said second position, said plunger means being operable to move toward said second position in response to increasing pressure of a sensed uid, lost motion means operable to move said snap element from one position of stability past said position of instability in response to movement of said plunger means from said first position to said second position and being operable to move said snap element from the other position of stability past said position of instability in response to movement from said second position to said first position, said lost motion means permitting free snap movement from the position of instability to said positions of stability when said snap element is operated thereby, and a switch which is opened and closed in response to movement of said snap element between said two positions of stability, said snap element assisting said spring means in resisting movement of said plunger means from said first position toward said second position and resisting said spring means when the latter causes movement of said plunger means from said second position toward said first position.

2. A switching device as set forth in claim 1 wherein said snap element is a flat metal spring pivoted at its ends in said body, said pivots normally being spaced apart a distance less than the unstressed spacing of said ends.

3. A switching device as set forth in claim 2 wherein said snap element is formed with a central opening through which said plunger means extends, and said lost motion means includes spaced opposed surfaces each operable to engage said snap element adjacent to said opening.

4. A switching device as set forth in claim 3 wherein said snap element is shaped to provide a substantially constant cross section between said ends.

5. A switching device as set forth in claim 4 wherein said switch includes a mobile contact mounted on said snap element adjacent to said central opening and spaced from said ends.

6. A switching device as set forth in claim 5 wherein said switch is open when said snap element is in a first of said positions of stability and opens and closes when said snap element is between said position of instability and said rst position of stability.

7. A switching device as set forth in claim 6 wherein said snap element has a negative spring rate as it approaches said position of stability which is greater than the spring rate of said spring means.

8. A switching device as set forth in claim 1 wherein said switch is open when said snap element is in a first of said positions of stability and opens and closes when said Snap element is between said position of instability and said rst position of stability.

l9. A switching device as set forth in claim 1 wherein said spring means and said snap element are sized so that said switch opens when the pressure of said sensed lluid drops below about 5 p.s.i. and closes when the pressure of said sensed fluid increases to pressures above about 35 p.s.i.

10. A switching device as set forth in claim 1 wherein stop means are provided to prevent movement of said plunger means beyond said second position.

11. A switching device as set forth in claim 10 wherein said plunger means includes an elastomeric diaphragm defining on one side at least part of an expansible chamber adapted to receive said sensed fluid, the side of said diaphragm opposite said chamber being substantially totally conned by said body and plunger means whereby said diaphragm is substantially completely supported against damage from excessive pressures in said chamber.

12. A pressure responsive switching device comprising a body formed with a tubular skirt extending to one end, a switch base in said body', said body and switch base being formed of electrically conductive material, a

support member formed of electrically insulating material in said body at each end of said switch base, said support members each closely fitting said skirt and being provided with axially extending surfaces inter-engaging mating surfaces on said switch base to radially support said switch base and prevent contact between said body and switch base, a pair of terminals mounted on one of said support members, one of said terminals being electrically connected to said switch base, a -tixed contact means mounted on said one support member connected the other of said terminals, a mobile contact means mounted on said switch base for movement into and out of contact with said xed contact, and pressure responsive plunger means operatively connected to move said mobile contact means into and out of engagement with said fixed contact means.

13. A switching device as set forth in claim 12 wherein said mobile contact means includes a at spring metal snap element, said snap element being pivoted at its ends on said switch base, said pivots being spaced apart a distance less than the unstressed spacing of said ends.

14. A switching device as set forth in claim 13 wherein said snap element is formed with a central opening, said plunger means extending through said opening and provides opposed surfaces each operable to engage said snap element adjacent to said opening, said opposed surfaces being spaced apart a distance substantially greater than the thickness of said snap element whereby a lost motion connection is provided for driving said snap element.

15. A switching device as set forth in claim 12 wherein said axially extending surfaces of said one support member are noncircular to provide a predetermined orientation of said switch base and one support member, and said axially extending surfaces of the other of said support members being substantially cylindrical.

References Cited UNITED STATES PATENTS 2,111,168 3/1938 Chansor 20G-83.8 2,197,473 4/ 1940 Jackson et al. 200e-83.5 2,230,770 2/ 1941 Van Almelo 200--83.2 3,109,908 11/1963 Clason 200-83.5 3,268,683 8/1966 Palmer 200-83.8 3,302,269 2/1967 Cooper et al. 20G-83.2 XR

ROBERT K. SCHAFFER, Primary Examiner J. R. SCOTT, Assistant Examiner 

